Removal of saturated carbonyls from
unsaturated nitriles by ph-controlled
distillation



United States Patent REMOVAL OF SATURATED CARBONYLS FROM UNSATURATEDNITRILES BY pH-CONTROLLED DISTILLATION Donald G. Stevens, Arthur F.Miller, and Gerald E. King, by The Standard Oil Company, a corporationof Ohio, assignee N0 Drawing. Original No. 3,185,636, dated May 25,1965, Ser. No. 174,292, Feb. 19, 1962. Application for reissue Sept. 22,1967, Ser. No. 675,264

2 Claims. (Cl. 203-45) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE A method for removing saturated carbonylsfrom acrylonitrile or methacrylonitrile by absorbing the reactionproducts from the amm o xidatifl'rt of an olefin in an aqueous solutionat a pH of 6.5 to 8.5, distilling the nitrile overhead with cvmzhydrins. The cyanhydrins are formed by the combination 0 thesaturated carbonyls and the hydrogen cyanide. The cyanhydrins in theoverhead are stabilized by maintaining a pH of 2 to 5 of the overhead bythe addition 0 an acid to the overhead and then distilling the nitrileoverhead from the acid stabilized cyanhydrin.

This invention relates to the purification of unsaturated nitriles suchas acrylonitrile and methacrylonitrile, to separate them from smallamounts of saturated carbonyl compounds.

When an olefin, such as propylene or isobutylcnc is reacted with ammoniaand oxygen to produce the corresponding unsaturated nitriles such asacrylonitrile or meth acrylonitrile, there is also produced substantialamounts of hydrogen cyanide and trace amounts of carbonyl compounds ofrelatively low molecular weight, such as acctaldehyde. propionaldehyde,acrolein, methacrolein, acetone, methyl ethyl kctone, etc.

The separation of the unsaturated carbonyl compounds, when they arepresent in trace amounts, presents no problem since these arehomo-polymerizable under the normal distillation conditions and do notrequire any treatment, treating step, or treating agent in order thatthey may be separated from the unsaturated nitrile. The acrolein, forexample, without treatment. readily dimerizes to the water-soluble dimerand does not go overhead when acrylonitrile is distilled but remains inthe waste water from the absorber.

The saturated carbonyl compounds, however, present troublesomepurification problems, because they do not polymerize. In accordancewith this invention, these saturated carbonyl compounds can beeffectively separated from the unsaturated nitrile so that the lattercan be recovered substantially free from saturated carbonyl compounds,i.e., less than a total of 300 parts per million maximum and generallywell under 100 parts per million.

In accordance with this invention, it has been discovered that if thereaction products from the ammoxidation of an olefin, ammonia andmolecular oxygen are absorbed in water, which is maintained at asubstantially neutral or slightly alkaline pH, such as a pH of 6.5 to8.5, the saturated carbonyl compounds combine wilh the excess hydrogencyanide to form the corresponding cyanhydrin. If the unsaturated nitrilerecovered from this aqueous solution is then kept in acid condition,i.e., as a pH of 5 or less, the cyanhydrins formed in the previous stepare stabilized and do not decompose to hydrogen cyanide till ReissuedJune 4, 1968 and the corresponding saturated carbonyl during thesubsequent purification steps. The stabilized cynnhydrins boil muchhigher than the unsaturated nitrilc and the latter can, therefore,effectively be distilled from them.

Unless these saturated carbonyls are combined in the form of thecyanhydrins under conditions leading to the conversion of substantiallyall of the carbonyls to the cyanhydrin form and the cyanhydrins thenstabilized and retained in this form, troublesome separation problemsarise. More particularly, the cyanhydrins decompose to hydrogen cyanideand the corresponding saturated cmbonyl under some operating conditions,and recombine to form the cyanhydrin under other conditions so that itis difficult to eliminate the carbonyls.

The conversion of the saturated carbonyls to the cyanhydrin bymaintaining neutral or slightly alkaline conditions can be accomplishedin any of a number of ways. The amount of ammonia in the reactorctllucnt can be adjusted so that a pH of 6.5 to 8.5 is maintained inabsorber water. If, however, the products are quenched with water at alower pH, the absorber solution can have its pH elevated by the additionof any alkaline compound, such as sodium carbonate. The means by whichthe absorbing solution is maintained within the pH range of 6.5 to 8.5is not critical and any method of accomplish ing it, as long as the pHrange is achieved, is contem lated within the invention.

The unsaturated nitrile is distilled from the aqueous absorbing solutionand any cyanhydrins which are removed overhead along with theacrylonitrile and then stabilized by the addition of an acid to theacrylonitrile stream. Any acid may be used, such as phosphoric acid,sulfuric acid, acetic acid, etc, so as to maintain the pH of theacrylonitrile within the range of 2 to 5.

In accordance with the invention, the acrylonitrile can be distilledoverhead as a pure product and all of the stabilized saturated carbonylsin the form of cyunhydrins remains in the bottoms from the fractionatingcolumn and can be continuously or periodically withdrawn.

The invention will be described hereinafter in conneo tion with thepurification of acrylonitrile, such as made by the reaction ofpropylene, ammonia, and air and more particularly, the purification ofacrylonitrile to separate it from saturated carbonyl of which acetone isone of the principal components and the most troublesome for separation.

Soichiomctric amounts of propylene, ammonia and air to formacrylonitrile are fed to a reactor containing an oxidation catalyst atan elevated temperature and the reactor effluent contains 3.99%acrylonitrile, 0.77% hy drogen cyanide and 0.02% acetone andacetaldchycle (about 0.5% based on the acrylonitrile). The balance isnitrogen from the air, unreacted oxygen, propylene and ammonia, togetherwith carbon dioxide, water and such other components which do not haveany effect upon the recovery process herein described.

The reactor efiluent, at a temperature of about 850 F., is quenched inwater maintained at a pH wllhin the range of 6 to 6.5 to neutralize anyunrcacted ammonia and the solution is fed to an absorbing column inwhich the acrylonitrilc is distilled overhead. In this column enoughsodium carbonate is added and, if required, to maintain the pH of 6.5 to6.8. The acrylonitrilc is distilled overhead from the aqueous solutiontogether with 0.15% of acetone and acetaldchyde, as cyanhydrins ofhydrogen cyanide based on the acrylonitrile, and immediately uponcondensation there is added sufi'icient phosphoric acid to maintain a pHof 4.8. The product is then further distilled, dried and finallypurified by distillation in the presence of phosphoric acid to maintaina pH of 5. The final product has less than parts per million of acetoneand acctaldehyde free or as the cyanhydrins.

We claim:

1. A process for purifying an unsaturated nitrile selected from thegroup consisting of acrylonitrile and methacrylonitrile, which iscontaminated by substantial amounts of hydrogen cyanide and traceamounts of saturated carbonyl compounds selected from the groupconsisting of acetaldehyde, propionaldehyde, [acrolein, methacrolein,]acetone, and methyl ethyl ketone, which comprises absorbing saidunsaturated nitrile in an aqueous solution at a pH of 6.5 to 8.5 wherebythe saturated carbonyls form cyanhydrins with the hydrogen cyanide,distilling as overhead the unsaturated nitrile with at least a part ofthe cyanhydrins from the absorbing solution, stabilizing the cyanhydrinsremaining in the unsaturated nitrile collected as overhead bymaintaining the unsaturated nitrile at a pH of 2 to 5 by the addition ofacid thereto, and distilling the unsaturated nitrile overhead from theacid stabilized cyanhydrins.

2. A process for purifying acrylonitrile which is contaminated bysubstantial amounts of hydrogen cyanide and trace amounts of acetone,which comprises adding sodium carbonate to an aqueous absorbing solutionof the abovementioned mixture to achieve apH of 6.5 to 8.5,

whereby the acetone forms a cyanhydrin with the hydrogen cyanide,distilling as the overhead the acrylonitrile with at least a part of thecyanhydrin from the aqueous solution, and stabilizing the cyanhydrin inthe acrylonitrile collected as overhead by adding an acid to theacrylonitrile to achieve a pH range of 2 to 5, and distilling theacrylonitrile overhead from the stabilized cyanhydrin.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS WILBUR L. BASCOMB, JR., Primary Examiner.

